The present invention relates to a continuous fermentation process for producing a heteropolysaccharide. Considerable interest in heteropolysaccharides produced by the bacterial fermentation of carbohydrates has been exhibited in recent years. Impetus has been given to the development of this interest by the discovery that certain heteropolysaccharides formed by biochemical synthesis have properties which permit their use as thickening agents for water used in secondary recovery operations carried out in the petroleum industry. It has been found that some of these materials added to water or brine in suitable concentrations produce viscous solutions which are relatively stable under the conditions which prevail in subsurface oil reservoirs. By utilizing a solution of controlled viscosity in place of or in addition to the water or brine normally employed in waterflooding projects, a favorable mobility ratio between the oil in the reservoir and the liquid used to displace it can be obtained. The tendency of the displacing medium to finger through highly permeable sections of the reservoir without displacing oil from the less permeable sections is greatly decreased. Viscous forces which normally reduce the displacement efficiency in portions of the reservoir through which the displacing medium actually passes are more readily overcome. As a result of these effects, the use of water or brine containing polysaccharide thickening agents generally permits the recovery of significantly greater quantities of oil during waterflooding than can be removed with water or brine alone.
A particularly effecting polysaccharide for use as a thickening agent during oil field waterflooding operations is the heteropolysaccharide produced by the action of bacteria of the genus Xanthomonas upon sugar, starches and similar carbohydrates. This material is more commonly known as "Xanthan gum". Studies and comparative tests have shown that this material, a polymer containing mannose, glucose, glucuronic acid salts, and acetyl radicals in a molar ratio of about 2:1:1:1 respectively, has much greater thickening power than dextran and similar polysaccharides and hence can be used in significantly lower concentrations than the other materials. It is effective in both fresh water and brine and has excellent high temperature stability characteristics. It is not precipitated or adsorbed to a significant extent upon contact with porous rock and sands commonly found in oil-bearing reservoirs. It exhibits little or no tendency to plug unconsolidatd sand reservoirs operations. The combination of all of these properties makes the heteropolymer formed by Xanthomonas organisms from carbohydrates considerably more attractive than other polysaccharides for use as water thickeners in secondary or tertiary recovery operations.
Although Xanthan gum has several uses in industrial application, primarily as a thickener in food processing and as a drilling "mud" in the oil industry, a new use has been developed in tertiary flooding of oil wells.
The best thickener known for use in tertiary flooding of oil wells is Xanthan gum, because it does not shear, clog the strata, or react with minerals underground. Xanthan gum has become the standard by which alternative compounds for this use are rated.
The heteropolysaccharide described above is normally produced by inoculating a medium containing from about 1 to about 5 percent by weight of a suitable carbohyrdrate, organic nitrogen sources, dipotassium hydrogen phosphate and appropriate trace elements with an organism of the genus Xanthomonas and then permitting the culture to incubate at about room temperature and under aerobic conditions for a period of about three to four days. Carbohydrates which may be employed in this manner include glucose, fructose, maltose, sucrose, lactose, galactose, soluble starch, e.g. corn starch and the like. Specific Xanthomonas organisms which may be used to produce heteropolysaccharides include Xanthomonas campestris, Xanthomonas phaseoli, Xanthomonas malvacearum, Xanthomonas translucens, Xanthomonas carotae, Xanthomonas hederaz, Xanthomonas papavericola, Xanthomonas begoniae, Xanthomonas incanae, Xanthomonas vasculorum and Xanthomonas vesicatoria. At the end of the incubation period, the crude polymer formed in the culture medium can be separated from the bacterial cells by centrifugation or filtration and can thereafter be isolated and purified by precipitating it with methanol, ethanol, acetone or a similar reagent. After drying, the heteropolymer is recovered as a light fluffy powder which may be slightly tinted by colored materials from the culture medium.
Conventional processes for making Xanthan gums involve either batch or continuous one or more stage processes. Although the conventional processes involve low capital cost, the cost of removal of cells in the product is high. Because the viscosity of the final product is high, the final product must be diluted to facilitate cell removal. Removal of the final product is generally effected by centrifugation or precipitation. During fermentation, the high viscosity of the polymer may prevent nutrients from getting to the cells, and therefore inhibit continued product formation. The yield of product, in dry weight, is based on the amount of original carbohydrate consumed. Total production time according to these processes is about 96 hours.
For many uses, a heteropolysaccharide of high salt, heat, and acid resistance is evaluated primarily on the basis of its viscosity characteristics. Since these viscosity characteristics are approximately the same for a crude product as for a refined product, where the same actual polymer concentration exists, it is economically feasible to produce a crude product. The term "crude" for this product is defined as a low-cost, bacteria-free, slightly off-white heteropolysaccharide polymer.